fix: Replace Prawn with Grover for PDF ticket generation

- Replace Prawn PDF generation with Grover (Chrome headless) for better compatibility
- Add HTML-based ticket template with embedded CSS styling
- Implement robust Grover loading with fallback to HTML download
- Add QR code generation methods to Ticket model
- Remove legacy TicketPdfGenerator service and tests
- Update PDF generation in TicketsController with proper error handling

The new implementation provides:
- Better HTML/CSS rendering for ticket layouts
- More reliable PDF generation using Chrome engine
- Fallback mechanism for better user experience
- Cleaner separation of template rendering and PDF conversion

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/models/ticket.rb

@@ -27,6 +27,29 @@ class Ticket < ApplicationRecord
     TicketPdfGenerator.new(self).generate
   end
 
+  # Generate QR code data for ticket validation
+  def to_qr_data
+    {
+      ticket_id: id,
+      qr_code: qr_code,
+      event_id: event&.id,
+      user_id: user&.id
+    }.compact.to_json
+  end
+
+  # Generate QR code as SVG
+  def generate_qr_svg
+    require 'rqrcode'
+    qrcode = RQRCode::QRCode.new(to_qr_data)
+    qrcode.as_svg(
+      offset: 0,
+      color: '000',
+      shape_rendering: 'crispEdges',
+      module_size: 4,
+      standalone: true
+    )
+  end
+
   # Price in euros (formatted)
   def price_euros
     price_cents / 100.0